performance data from commercial equipment, there is considerable uncertainty about the practical implications of operating and maintaining these systems. For this reason, the ratings for complexity, ease of monitoring, and crew impact given in Table 4-1 are negative. Further discussion of electric pulse and pulse plasma systems is provided in Appendix F.

The committee judged four candidate treatments—ultraviolet, acoustic, magnetic, and deoxygenation—to be inappropriate for shipboard treatment of ballast water since these methods have not been proven effective for a broad range of freshwater and marine organisms. Two other techniques, biological treatment and antifouling coatings, were not evaluated in any detail by the committee because their disadvantages clearly outweighed their advantages.

Ultraviolet treatment is effective in destroying micro-organisms, but not in removing or inactivating higher organisms and cyst or spore stages of protozoa, fungi, microalgae (including dinoflagellates), and macroalgae (AQIS, 1993). In addition, the effectiveness of ultraviolet disinfection is greatly reduced in water containing suspended matter due to absorption and screening effects.

The use of underwater acoustic energy sources (including ultrasonics) to destroy or deter various aquatic species has been demonstrated on a laboratory scale, but the effects appear to be dependent on frequency and species, with no general effectiveness over the wide range of organisms likely to be found in ships' ballast tanks. Ultrasonic frequencies around 20 kHz kill or inactivate bacteria and fungi but not higher organisms. Lower frequency acoustic signals deter fish, although the deterrent frequency is species dependent. High-intensity acoustic sources can shatter the shells of juvenile zebra mussels and lead to the lethal disintegration of veligers. Further details of investigations of acoustic treatment are given in Appendix F.

Magnetic treatment of water on a laboratory scale has been effective against calcareous shell-forming invertebrates, notably zebra mussels, and has undergone significant development for this application. However, the treatment mechanism is not understood and the effect of magnetic treatment on marine